1. Field of the Invention
The present invention relates to a liquid concentration measuring device that measures an alcohol concentration and the like.
2. Description of Related Art
Recently, low-pollution alcohol blended gasoline has been attracting attention as fuel for an automobile. Such the mixed gasoline has the optimum air-fuel ratio different from that of fuel consisting of only gasoline. Therefore, it is important to measure a content of the alcohol in the mixed gasoline, i.e., an alcohol concentration, in order to perform fuel injection control to achieve the optimum air-fuel ratio for the mixed gasoline.
Conventionally, there has been proposed a control device of an engine that measures an alcohol concentration and that controls a fuel injection quantity (for example, refer to Patent document 1: Japanese patent application publication No. 2006-322401). Generally, the alcohol has volatility lower than that of the gasoline. Therefore, the fuel injection quantity is corrected to be increased as the alcohol concentration increases and as temperature of the alcohol blended gasoline decreases.
In order to accurately measure the alcohol concentration, it is desirable to use a physical constant having a comparatively high change ratio. Therefore, a capacitive alcohol concentration sensor that senses a change in a relative dielectric constant is proposed (for example, refer to Patent document 2: PCT international patent application Japanese translation No. H5-507561 and Patent document 3: Japanese patent application publication No. H6-3313).
There is a possibility that the alcohol concentration sensor causes a stuck failure abnormality, in which an output becomes a fixed value, due to an aging change and the like. If such the stuck failure abnormality occurs, a deviation arises between the optimum air-fuel ratio and the correction of the fuel injection quantity.
Therefore, there is proposed a diagnostic device that determines an abnormality by detecting a deviation of a correction value based on air-fuel ratio feedback (for example, refer to Patent document 4: Japanese examined patent application publication No. H6-94822). The diagnostic device can determine occurrence of the abnormality in a system of an internal combustion engine.
There is proposed another abnormality diagnostic device that determines an abnormality in an alcohol concentration sensor itself (for example, refer to Patent document 5: Japanese examined utility model application publication No. H6-614).
The diagnostic device described in Patent document 4 can determine the occurrence of the abnormality in the system of the internal combustion engine. However, the diagnostic device cannot determine a failure of the alcohol concentration sensor itself. It is because, when an abnormality occurs in the system of the internal combustion engine, the cause of the abnormality is not limited to the failure of the alcohol concentration sensor, but rather, there can be many kinds of causes of the abnormality such as a failure of an injector, a failure of an air-fuel ratio sensor, leakage of intake air in an intake system and a failure of a fuel pump. Originally, the correction value based on the air-fuel ratio feedback contains variations of parts and a variation of the engine. Therefore, it is difficult to correctly find the cause of the variation of the correction value. Therefore, even if the abnormality of the system is determined, there is a possibility that a time necessary to specify the failure lengthens when the failure is specified in a repair shop or the like. As a result, there is a concern that a service level for a user lowers.
The abnormality diagnostic device described in Patent document 5 detects the abnormality in the sensor by monitoring a change in a sensor output. Therefore, the device cannot handle the stuck failure abnormality, in which the output from the sensor becomes a fixed value.